At first I thought it was referring to an exemption from the Injunction, because that seems to be part of Mostin's concern with the whole situation: Soneillon is exempt, Shomei may be as well, and the Wyrish collegium is potentially caught between them.

However, rereading this:

Devas and archons appeared all around him, summoned by his thought, but her presence paralyzed them; they would not strike her, only worship her. He smote her repeatedly, but her exemption protected her. Her will recommenced, unleashing a cyclone of hellfire focused on herself which could not touch him, but which slowly burned the garland to ash.

He spoke a holy word; again, exemption sustained her.

Her exemption seems to protect her from a holy word and from smite evil. Perhaps it is some benefit conferred or realized from her relationship to Eadric and to the Sela or maybe it arises out of her rejection of a purely adversarial paradigm. I don't know. As a practical matter, it seems that she is highly resistant or immune to effects which would normally be effective against fiends or those of strong evil alignment. In other words, she is a fiend who is exempt from some of the normal perils of fiend-dom.

From her statblock (I am using my phone, can't copy the text and the link so easily)

Doubly Exempt (Ex): Shomei may ignore any alignment-based effects which might adversely affect her due to her subtypes; conversely, she may choose to be affected by any alignment-based effects which would provide her with a benefit. Devils and other lawful evil outsiders automatically recognize Shomei's status and their initial attitude toward her is always friendly; evil feys and suborned devils with the [Green] subtype treat her likewise. Divinations which attempt to discern information pertaining to Shomei's alignment always fail.

You can search for a few sentences from that for the full spectacularness :-)

D&D, frankly, is the most fun when you get your ass handed to you but you still manage to find away to come out on top of the pile of corpses, looking like a typical Conan novel cover. - joachim

As detailed in her writeup in the Legacy thread where Sep used to post crunch, Shomei is Doubly Exempt; this means she may ignore anything that would affect her due to her subtypes, or allow them as she sees fit, as well as being immune to any attempt to divine her alignment. It's something the Adversary gave her a while back when she became a devil.

Nwm and Ortwine stood waiting before the fortified gates at Galda, and watched as the rider approached from the south. She, her horse and her harness were caked in blood, ichor and entrails so thick that the muck might need to be scraped clear with a trowel rather than washed away; her visage was altogether terrifying.

As she approached, the Preceptor noticed that she carried another with her: a diminutive figure who clung desperately to her waist, barely able to remain upright in the saddle. She reined in, reached behind her, and lowered him gently to the ground. Her small companion shivered and stood unsteadily.

“Hello, Nwm,” Nehael said, “Ortwine. Teppu is tired, and I think he’d like a bath. Where is Eadric?”

Ortwine looked at Nwm and raised an eyebrow.

*

“Eadric is very popular with the ladies, these days, Nehael.” Ortwine gave a caustic smile. “And I must say, red rather suits you; I can see that it is also a practical color.”

They sat around a campfire: one of hundreds which burned in the encampment. The goddess had acquiesced to a cantrip to clean her of the foul-smelling gore which had clung to her, but which had seemed not to perturb her in any way; it was, in fact, for Ortwine’s benefit that she had agreed. Teppu was wrapped in a blanket, asleep.

“I had hoped to speak with him; to discuss the reconquest,” she threw off her boots.

Ortwine cast a sidelong glance at Nwm, who shrugged.

“Might I assume that you lack the prescience of your previous sister-avatar; now, your mother-deity?” Nwm inquired. “I am unaccustomed to explaining anything to Nehael; usually the information flows in the other direction.”

“I slay, Nwm,” she said simply. “This is the persona that you get: I make no apology for it. It is necessary. I don’t have time for magic or plots or webs. I am the counterpoint which Uedii must chant to contain the corruption; her image reflected through the Eleos: the enlightened, engaged, dynamic face of compassion. I am unsentimental, and occasionally ugly on the surface. Nor am I as Tree-ish as my former self; actually, I prefer horses.”

She lay down on her back in the wet earth and looked upwards. The Follower was soaring in the east, flickering through the smoke in the air. Some time passed before Nwm spoke again.

“You are aware that Eadric is currently being held by Shomei the Infernal?” The Preceptor queried.

“Well, I imagine I might have been, had I thought about it.” She thought about it. “I see. I suppose I could go and talk to her.”

“Could?” Ortwine gave a quizzical look.

Nehael raised her head, leant on an elbow, and smiled. “Eadric is confused, Ortwine. It is his defining feature. He gets himself into these situations; I’m not really convinced that my becoming involved at this stage would help. He should have followed my advice, and simply exercised compassion.”

“In which specific instance?” Nwm asked.

“He shouldn’t have censured Shomei, Nwm. It didn’t help. Really, he just lost his temper and became offended and pious. It’s always been an issue with him.”

“Mostin is working on a solution.”

“Yes. Mostin may aggravate the problem further,” Nehael remarked.

“And this assumes that Soneillon does not become unhinged in the interim,” Nwm added.

“Ahh, Soneillon,” she lay back down. “Another situation.”

Ortwine stared hard at her. “For an avatar of compassion, you seem very free in your criticism of those absent.”

“I would say the same to him – or her – were either here, Ortwine. As you have rightly implied, malicious gossip is incompatible with my nature. And frankly, the march of Dhatri’s host and the Embassy are of more concern to me at present than Eadric’s convoluted emotional life. I put an arrow in the latter earlier today; she knows I’m here well enough.”

“Then that is some good news.” Nwm grunted approvingly.

“She will not make the same mistake again.”

“By the Embassy, I assume you are referring to Kaalaanala’s final effluxion,” Ortwine sighed. “And each time I say that name I am nervous; in case I draw her perception to me.”

“The Trees protect you from that faculty here, Ortwine.”

The sidhe gave a stony look. “Had the Trees here been more comprehensive in their protection – and not allowed dreaming demonesses and exempt devils to penetrate their cordon - then this entire fracas might have been avoided. I think we may have placed too much faith in their effectiveness in protecting the Ahma’s moral fibre.”

“On the last count, I am inclined to agree.”

“So will you speak to Shomei?” Nwm inquired.

“Well. Are you asking for my intercession in this?”

“I don’t know,” Nwm admitted. “Should I?”

“Probably not,” Nehael replied.

“Then I suppose I won’t,” Nwm sighed. “But if I had, what would you have done?”

“Nothing,” Nehael smiled. “Which is the best that can be done at the moment. Ask me in a few days – things will probably unravel even more before they come together again. ”

“A prescience?” Nwm asked archly.

“Call it what you like,” Nehael shrugged. “I experience it as a vague notion. And today was too much for this one; he is too gentle.”

The goddess stood and removed her cloak. She folded it and placed it under Teppu’s head.

**

Eadric did not see Shomei – who had exited the cottage in order to perform conjurations, and sealed it behind her – for the entirety of the next day. After quickly becoming bored, he ventured forth to wander alone in obscure and musty corridors within the limitless repository which was Hell’s library. Ruddy candles burned with infinite slowness in deep sconces, barely illuminating the interior. It was eerily silent and – except for the occasional tome which itself exhibited some sign of sapience in addition to its malignancy – there was no question within the mind of the Ahma that he was alone.

Eadric was not a scholar; or rather, he had never had the time to pursue his scholarly interests: the art of war had demanded most of his attention throughout his life. The weight of infernal knowledge oppressed him, but more by virtue of its sheer volume than by its evil content. He considered the magnitude of Shomei’s commitment to the task of knowing the library; surely she must have read only the minutest fraction of the books contained within it. It seemed an impossible undertaking to master even its geography; to familiarize oneself with its contents would take a life’s age of the universe, or more.

It did not take him long to become lost, despite – what he had been sure – were his own meticulous precautions to the contrary regarding his bearing and distance from Shomei’s study. After a brief period of anxiety – during which he considered that his aimless wandering might, in fact, be his eternal lot – the Ahma determined that he would climb – the notion of ascent being comfortable and familiar to his inner aesthetic. Whenever a staircase – whether a narrow spiral, steep ladder, or wide companionway with sweeping balustrades – presented itself to him, he would eagerly scale it. At times, he would backtrack in frustration: his path would lead to a hidden nook, a suite of chambers or dark, diabolic cloisters with no other exit, and he would search out some new way. He entertained no notion of destination in his efforts, except up. Yet the light became no brighter; the atmosphere no less oppressive. There was no relief to be had, except in the act of ascent itself; a metaphor which struck him as particularly apt, given the nature of his hostess – or gaoler.

After what must have been many hours – all sense of time having long since left him – Eadric stumbled upon an archway within which a grate of adamant bars had been set. Dire runes were carved in warning above the threshold; symbols which, although they posed no threat to him, would have slain any devil of lesser stature who might have approached them. He looked at the bars: no keyhole or aperture of any kind was present. Peering through the grate, only darkness was present beyond. Eadric ran his fingers around the archway, searching for some secret mechanism. Nothing.

He illuminated the space beyond with daylight. A narrow tunnel, extending ahead as far as he might see.

Mustering all of his strength, he gripped the grate and tore it away from the archway, placing it ruefully against the adjacent wall, conscious that he had committed some gross act of vandalism against the integrity of the place – then berating himself for entertaining any notion of guilt in the context of his current predicament. Lighting the passageway at intervals, he proceeded for a hundred yards until he came upon another archway – this time unblocked by gate or door. A sound threatened to overwhelm, until he recognized it. Some trap had been triggered; a holy word of great power. Eadric gave an ironic half-smile; fiendish interlopers – not the Ahma – had been on the mind of whoever had set the device: a barely-visible glyph which throbbed in the keystone above.

He entered into a low chamber perhaps ten feet on a side, and illuminated it. On shelves or chained to the walls were books with tarnished covers; they had been neglected and forgotten for many epochs. Ancient books. Forbidden books. Books whispering secrets best left untold. He opened one, and thumbed its metalline pages – Meditations on Radiance; and then another – Divining the Light; and then another – The World of Men to Come. He tilted his head.

They were celestial books, penned by great devils – then seraphs and other episemes – before the Fall.

He sat, and began to read.

**

Soneillon hovered high in the skies above the Academy, beyond the compass of the Hazel-ludja, and gazed at the shifting patterns around the scion. She was hidden – more effectively, she knew, than the fallen celestials who had come to spy upon her at Deorham – but was, herself, unable to penetrate the layers around the Tree below. A nest of hemi-demiplanes, through which a tortuous path wound to Shomei’s cottage: invulnerable to her magic and sight. The concursion which was the library’s ‘front door.’

There were many devils in the skies below her; of that, there could be no doubt. But they remained invisible; their numbers and type unknown. Six more days must elapse before Mostin could complete his arcane equations; a formula which would incorporate only herself, Mulissu and Nwm: the Alienist had indicated that he trusted no other – including Ortwine, whose duty to the Hazel was suspect – to be part of it.

She scowled, and retreated to Deorham; she considered that, were she to abandon it and Shomei to locate her beyond the stronghold , that some force brought against her might overwhelm her and deprive her of the Urn. Extinction was of no particular concern to her, but being bound – by Shomei – remained a possibility, however remote. The Infernalist would need a sizeable ritual pool in order to guarantee success, and would need time herself to devise a suitable rite – and some safe location in another world, from which it could be conducted.

Mostin had elected Sisperi as his venue. But Mostin might fail, whatever his mote-thing told him.

Soneillon considered the time she had before the test came. She allowed her anger to subside, and gave thought to entrenchment: should it become necessary, it would be as well to be prepared.

The demoness began in earnest to fortify both herself and Kyrtill’s Burh with powerful spells.

**

Shomei sat by the fire, reading, when Eadric entered. His route to her study was not something he could accurately recall; there was no doubt in his mind that she had guided him back by some art.

She raised her head as he entered. “Did you find what you were looking for?”

Or maybe that section wasn't just the stuff written by fallen angels... maybe heaven and hell are connected through their libraries! Eadric was in a corner of Oronthon's library and he never even knew how close he was to meeting the guy first hand!

After all, everything and everywhen is connected in L-space. Just watch out for monke-*